1. Field of the Invention
The present invention relates to an acceleration sensor module capable of performing sensitivity adjustment.
2. Description of the Related Art
Conventionally, as an acceleration sensor using piezoelectric ceramics, for example, the sensor described in Japanese Patent No. 2780594 has been known. Since such a sensor does not have an adjustment method for adjusting an individual sensor to a targeted sensitivity (voltage output in the case of applying a specified acceleration) or has difficulty in adjusting an individual sensor to a targeted sensitivity, it has a sensitivity tolerance between approximately xc2x15 and xc2x115% due to variations in manufacturing and materials.
However, a high-performance sensor having a sensitivity tolerance of less than xc2x15%, such as an acceleration sensor used in servo control of an MR-head for a Hard Disc Drive, is sometimes required. An ordinary procedure to meet the demand is to adjust the gain of an amplifier connected to the subsequent stage of the sensor. Consequently, in order to provide a sensor adjusted with high precision, a module having an amplifier therein is provided even if it is not necessary for the acceleration sensor characteristics itself, and the sensor is put into commercial production after adjustment of the gain is performed. As a result, this leads not only to increase in the size of the module, but also to an increase in cost.
Accordingly, it is an object of the present invention to provide an acceleration sensor module capable of performing sensitivity adjustment in a minimum arrangement without containing an amplifier.
In order to solve the above problems, an acceleration sensor module of the present invention includes a printed wiring board, and an acceleration sensor and a trimmable resistor, which are disposed on the printed wiring board, in which at least one terminal of the acceleration sensor and at least one terminal of the trimmable resistor are respectively connected via printed wiring to at least one external connection terminal on the printed wiring board to be connected to an external circuit.
The acceleration sensor and the trimmable resistor are mounted on the printed wiring board, and the terminal of the acceleration sensor and the terminal of the trimmable resistor are respectively separately connected to the external connection terminal disposed on the printed wiring board through the printed wiring. One of the terminals of the acceleration sensor and one of the terminals of the trimmable resistor may be connected to the external connection terminal by making a common connection in the circuit.
The resistance value of the trimmable resistor is adjusted by laser trimming or the like so as to adjust the sensitivity of the acceleration sensor, so that variations in the sensitivity can be held within a targeted tolerance. Since additional components such as an amplifier are not mounted on the printed wiring board on which the acceleration sensor and the trimmable resistor are mounted, miniaturization can be achieved, and moreover, significantly low-cost manufacturing is possible.
Although the trimmable resistor may be, for example, an individual component which is mounted on the printed wiring board by soldering or the like, it may also be a film-formed resistor, that is, a resistor film which is formed on or within the printed wiring board directly by a printing method or the like. In short, any type of resistor is usable as long as the adjustment of resistance values can be performed by laser trimming or the like.
As an acceleration sensor, a sensor using a semiconductor may be employed. According to an aspect of the present invention, it is preferable to use a piezoelectric acceleration sensor, wherein a piezoelectric ceramic element converts an acceleration into electrical signals and detects the acceleration. In this case, since a chip type of sensor can be formed, this contributes to miniaturization of the module.
According to another aspect of the present invention, it is preferable to connect a temperature compensation capacitor in parallel to the acceleration sensor in the vicinity of the acceleration sensor on the printed wiring board. The output voltage of the piezoelectric ceramic element is influenced by the ambient temperature in the piezoelectric acceleration sensor. However, it is possible to obtain an output voltage of the acceleration sensor which is hardly subjected to the influence of the ambient temperature by connecting a capacitor having appropriate capacitance and temperature characteristics in parallel with the acceleration sensor and in the vicinity of a place on the printed wiring board where the temperature is substantially the same as that of the acceleration sensor.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.